The present invention relates to a combustion burner of fine coal powder, wherein the fine coal powder is transported by an air flow, and a combustion apparatus of fine coal powder using same. Particularly, the present invention relates to a combustion burner for burning fine coal powder and a combustion apparatus of fine coal powder, both of which are preferable for decreasing concentration of nitrogen oxide (hereinafter, called as NOx) and unburned component in ashes.
Generally, suppression of NOx, which is generated in combustion, becomes a problem for combustion burners. Particularly, coal has a larger nitrogen content in comparison with gaseous fuels and liquid fuels.
Therefore, decreasing the amount of NOx generated in operation of the fine coal powder combustion burner is more important than the cases of the gaseous fuels and liquid fuels. Most of the NOx generated in burning coal (fine coal powder) is NOx (fuel NOx), which is generated by oxidizing nitrogen components contained in the coal.
Hitherto, various burner structures and combustion methods have been studied for decreasing the NOx. One of the effective combustion methods is a method (two stage combustion method) for burning the coal completely by supplying a deficient amount of air for complete combustion of the fine coal powder from the fine coal powder burner, and then, supplying additional air to make the amount of air sufficient for complete combustion in the downstream of the fine coal powder burner.
One other method is a method utilizing a reducing reaction of NOx, which is activated when oxygen concentration is low, by forming a region having a low oxygen concentration in flame. For instance, JP-A-1-305206 (1989), JP-A-3-211304 (1991), JP-A-3-110308 (1991), and others disclosed a method for burning coal completely by forming flame (reducing flame) having a low oxygen concentration atmosphere, and a structure, wherein a fine coal powder nozzle for transporting fine coal powder by an air flow is set at a center, and air nozzles for injecting air are arranged outside around the fine coal powder nozzle.
In accordance with these low NOx burners, a region having a low oxygen concentration is formed in flame, and NOx is reduced to harmless nitrogen molecules by generating NOx reducing materials such as ammonia and hydrogen cyanide from the nitrogen components contained in the fine coal powder in the reducing flame region. That is, the amount of NOx generated in the flame is decreased, because the NOx is reduced to nitrogen molecules.
In the case of using the two stage combustion method, the amount of air supplied to the fine coal powder burner is smaller than the amount of air necessary for complete combustion of the fine coal powder. Accordingly, air (air for second stage combustion) is further supplied in the downstream of the fine coal powder burner for complete combustion. Therefore, the combustion apparatus for the two stage combustion method must be provided with a space for mixing the air for second stage combustion with the fine coal powder.
For instance, a boiler furnace (combustion apparatus) for 1000 MW power generation requires to ensure approximately five meters in height as a mixing space for second stage combustion air per sixty meters in height of the furnace. In accordance with a single stage combustion method, wherein all the air for combustion is supplied by the fine coal powder burner, the mixing space can be omitted, and the height of the furnace can be decreased. However, in the case of the single stage combustion method, the air for combustion is readily mixed with the fine coal powder flow, and, even if the low NOx burner is used, the releasing amount of NOx tends to increase significantly in comparison with the case of the two stage combustion method. If a strong swirl is given to the combustion air in order to suppress mixing the fine coal powder with the combustion air supplied from the air nozzle, the fine coal powder is not mixed completely with the combustion air even in the downstream region of the burner, and the amount of unburned component in the ashes is increased.